Image projection system with a polarizing beam splitter

ABSTRACT

An image projection system has a wire grid polarizing beam splitter which functions as both the polarizer and the analyzer in the system. A light source produces a source light beam directed at the beam splitter which reflects one polarization and transmits the other. A liquid crystal array is disposed in either the reflected or transmitted beam. The array modulates the polarization of the beam, encoding image information thereon, and directs the modulated beam back to the beam splitter. The beam splitter again reflects one polarization and transmits the other so that the encoded image is either reflected or transmitted to a screen. The beam splitter can be an embedded wire grid polarizer with an array of parallel, elongated, spaced-apart elements sandwiched between first and second layers. The elements form a plurality of gaps between the elements which provide a refractive index less than the refractive index of the first or second layers.

[0001] This application is a continuation-in-part of U.S. Ser. No.09/363,256 filed Jul. 28, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an image projection systemoperable within the visible spectrum which includes a polarizing beamsplitter which reflects one linear polarization of light and transmitsthe other. More particularly, the present invention relates to such animage projection system with a beam splitter that is comprised of aplurality of elongated, reflective elements which are disposed on asubstrate in such a way to reduce geometric distortions, astigmatismand/or coma in the resulting light beam, and/or which are embedded orotherwise configured to protect the elements.

[0004] 2. Related Art

[0005] Polarized light is necessary in certain applications, such asprojection liquid crystal displays (LCD). Such a display is typicallycomprised of a light source; optical elements, such as lenses to gatherand focus the light; a polarizer that transmits one polarization of thelight to the liquid crystal array; a liquid crystal array formanipulating the polarization of the light to encode image informationthereon; means for addressing each pixel of the array to either changeor retain the polarization; a second polarizer (called an analyzer) toreject the unwanted light from the selected pixels; and a screen uponwhich the image is focused.

[0006] It is possible to use a single polarizing beam splitter (PBS) toserve both as the first polarizer and the second polarizer (analyzer).If the liquid crystal array is reflective, for example a Liquid CrystalOn Silicon (LCOS) light valve, it can reflect the beam that comes fromthe polarizer directly back to the polarizer after encoding the image bymodifying the polarization of selected pixels. Such a system wasenvisioned by Takanashi (U.S. Pat. No. 5,239,322). The concept waselaborated by Fritz and Gold (U.S. Pat. No. 5,513,023). These similarapproaches would provide important advantages in optical layout andperformance. Neither, however, has been realized in practice because ofdeficiencies in conventional polarizing beam splitters. Thedisadvantages of using conventional polarizing beam splitters inprojection liquid crystal displays includes images that are not bright,have poor contrast, and have non-uniform color balance or non-uniformintensity (due to non-uniform performance over the light cone). Inaddition, many conventional polarizing beam splitters are short-livedbecause of excessive heating, and are very expensive.

[0007] In order for such an image projection system to be commerciallysuccessful, it must deliver images which are significantly better thanthe images provided by conventional cathode ray tube (CRT) televisiondisplays because it is likely that such a system will be more expensivethan conventional CRT technology. Therefore, the image projection systemmust provide (1) bright images with the appropriate colors or colorbalance; (2) have good image contrast; and (3) be as inexpensive aspossible. An improved polarizing beam splitter (PBS) is an importantpart of achieving this goal because the PBS is a limiting componentwhich determines the potential performance of the display system.

[0008] The PBS characteristics which significantly affect the displayperformance are (1) the angular aperture, or the f-number, at which thepolarizer can function; (2) the absorption, or energy losses, associatedwith the use of the PBS; and (3) the durability of the PBS. In optics,the angular aperture or f-number describes the angle of the light conewhich the PBS can use and maintain the desired performance level. Largercones, or smaller f-numbers, are desired because the larger cones allowfor more light to be gathered from the light source, which leads togreater energy efficiency and more compact systems.

[0009] The absorption and energy losses associated with the use of thePBS obviously affect the brightness of the system since the more lightlost in the optics, the less light remains which can be projected to theview screen. In addition, the amount of light energy which is absorbedby the polarizer will affect its durability, especially in such imageprojection systems in which the light passing through the optical systemis very intense, on the order of watts per square centimeter. Light thisintense can easily damage common polarizers, such as Polaroid sheets. Infact, the issue of durability limits the polarizers which can be used inthese applications.

[0010] Durability is also important because the smaller and lighter theprojection system can be made, the less expensive and more desirable isthe product. To accomplish this goal, however, the light intensity mustbe raised even higher, further stressing the PBS, and shortening itsuseful life.

[0011] A problematic disadvantage of conventional PBS devices is poorconversion efficiency, which is the primary critical performance factorin displays. Conversion efficiency is a measure describing how much ofthe electrical power required by the light source is translated intolight intensity power on the screen or panel that is observed by peopleviewing it. It is expressed as the ratio of total light power on thescreen divided by the electrical power required by the light source. Theconventional units are lumens per watt. A high ratio is desirable for anumber of reasons. For example, a low conversion efficiency will requirea brighter light source, with its accompanying larger power supply,excess heat, larger enclosures and cabinet, etc. In addition, all ofthese consequences of low conversion efficiency raise the cost of theprojection system.

[0012] A fundamental cause of low conversion efficiency is poor opticalefficiency, which is directly related to the f-number of the opticalsystem. A system which has an f-number which is half the f-number of anotherwise equivalent system has the potential to be four times asefficient in gathering light from the light source. Therefore, it isdesirable to provide an improved polarizing beam splitter (PBS) whichallows more efficient harvesting of light energy by offering asignificantly smaller potential f-number (larger angular aperture), andtherefore increases the conversion efficiency, as measured inlumens/watt.

[0013] There are several reasons for the poor performance ofconventional polarizing beam splitters with respect to conversionefficiency when they are used as beam splitters in projection systems.First, current beam splitters work poorly if the light does not strikethem at a certain angle (or at least, within a narrow cone of anglesabout this principal angle of incidence). Deviation of the principal rayfrom this angle causes each type of polarizing beam splitter to degradethe intensity, the purity of polarization, and/or the color balance.This applies to the beam coming from the light source as well as to thebeam reflected from the liquid crystal array. This principal angledepends upon the design and construction of the PBS as well as thephysics of the polarization mechanism employed in these various beamsplitters. Currently available polarizing beam splitters are not capableof operating efficiently at angles far from their principal polarizingangles in the visible portion of the electromagnetic spectrum. Thisrestriction makes it impossible to implement certain promising opticallayouts and commercially promising display designs.

[0014] Even if the principal ray strikes the polarizer at the best anglefor separating the two polarizations, the other rays cannot diverge farfrom this angle or their visual qualities will be degraded. This is aserious deficiency in a display apparatus because the light striking thepolarizer must be strongly convergent or divergent to make efficient useof the light emitted by typical light sources. This is usually expressedas the f-number of the optical system. For a single lens, the f-numberis the ratio of the aperture to the focal length. For optical elementsin general, the F-number is defined as

F/#=1/(2 n sin Θ)

[0015] where n is the refractive index of the space within which theoptical element is located, and Θ is the half cone angle. The smallerthe F-number, the greater the radiant flux, Φ_(c), collected by thelens, and the more efficient the device will be for displaying a brightimage. The radiant flux increases as the inverse square of the F/#. Inan optical train, the optical element with the largest F/# will be thelimiting factor in its optical efficiency. For displays usingtraditional polarizers, the limiting element is nearly always thepolarizer, and thus the PBS limits the conversion efficiency. It wouldclearly be beneficial to develop a type of PBS that has a smaller F/#than any that are currently available.

[0016] Because traditional polarizers with small F/#s have not beenavailable, designers typically have addressed the issue of conversionefficiency by specifying a smaller, brighter light source. Such sources,typically arc lamps, are available, but they require expensive powersupplies that are heavy, bulky, and need constant cooling while inoperation. Cooling fans cause unwanted noise and vibration. Thesefeatures are detrimental to the utility of projectors and similardisplays. Again, a PBS with a small F/# would enable efficient gatheringof light from low-power, quiet, conventional light sources.

[0017] Another key disadvantage of conventional polarizing beamsplitters is a low extinction, which results in poor contrast in theimage. Extinction is the ratio of the light transmitted through thepolarizer of the desired polarization to the light rejected of theundesired polarization. In an efficient display, this ratio must bemaintained at a minimum value over the entire cone of light passingthrough the PBS. Therefore, it is desirable to provide a polarizing beamsplitter which has a high extinction ratio resulting in a high contrastimage.

[0018] A third disadvantage of conventional polarizing beam splitters isa non-uniform response over the visible spectrum, or poor colorfidelity. The result is poor color balance which leads to furtherinefficiency in the projection display system as some light from thebright colors must be thrown away to accommodate the weaknesses in thepolarizing beam splitter. Therefore, it is desirable to provide animproved polarizing beam splitter that has a uniform response over thevisible spectrum, (or good color fidelity) giving an image with goodcolor balance with better efficiency. The beam splitter must beachromatic rather than distort the projected color, and it must notallow crosstalk between the polarizations because this degrades imageacuity and contrast. These characteristics must apply over all portionsof the polarizer and over all angles of light incidence occurring at thepolarizer. The term spathic has been coined (R. C. Jones, Jour. OpticalSoc. Amer. 39, 1058, 1949) to describe a polarizer that conservescross-sectional area, solid angle, and the relative intensitydistribution of wavelengths in the polarized beam. A PBS that serves asboth a polarizer and analyzer must be spathic for both transmission andreflection, even in light beams of large angular aperture.

[0019] A fourth disadvantage of conventional polarizing beam splittersis poor durability. Many conventional polarizing beam splitters aresubject to deterioration caused by excessive heating and photochemicalreactions. Therefore, it is desirable to provide an improved polarizingbeam splitter that can withstand an intense photon flux for thousands ofhours without showing signs of deterioration. In addition, it isdesirable to provide a polarizing beam splitter that is amenable toeconomical, large scale fabrication.

[0020] The need to meet these, and other, criteria has resulted in onlya few types of polarizers finding actual use in a projection system.Many attempts have been made to incorporate both wide angular apertureand high fidelity polarization into the same beam splitting device. Therelative success of these efforts is described below. Thin filminterference filters are the type of polarizer cited most frequently inefforts to make a polarizing beam splitter that is also used as ananalyzer. MacNeille was the first to describe such a polarizer that waseffective over a wide spectral range (U.S. Pat. No. 2,403,731). It iscomposed of thin-film multi-layers set diagonally to the incident light,typically within a glass cube, so it is bulky and heavy compared to asheet polarizer. What is more, it must be designed for a single angle ofincidence, usually 45°, and its performance is poor if light is incidentat angles different from this by even as little as 2°. Others haveimproved on the design (e.g. J. Mouchart, J. Begel, and E. Duda, AppliedOptics 28, 2847-2853, 1989; and L. Li and J. A. Dobrowolski, AppliedOptics 13, 2221-2225, 1996). All of them found it necessary to seriouslyreduce the wavelength range if the angular aperture is to be increased.This can be done in certain designs (U.S. Pat. Nos. 5,658,060 and5,798,819) in which the optical design divides the light intoappropriate color bands before it arrives at the polarizing beamsplitter. In this way, it is possible to reduce the spectral bandwidthdemands on the beam splitter and expand its angular aperture, but theadditional components and complexity add significant cost, bulk, andweight to the system.

[0021] Even so, these improved beam splitter cubes are appearing on themarket, and are currently available from well known vendors such asBalzers and OCLI. They typically offer an F/# of f/2.5-f/2.8, which is asignificant improvement over what was available 2 years ago, but isstill far from the range of F/1.2-F/2.0 which is certainly within reachof the other key components in optical projection systems. Reachingthese f-numbers has the potential to improve system efficiency by asmuch as a factor of 4. They would also enable the projection displayengineer to make previously impossible design trade-offs to achieveother goals, such as reduced physical size and weight, lower cost, etc.

[0022] In a technology far from visible optics, namely radar, wire gridshave been used successfully to polarize long wavelength radar waves.These wire grid polarizers have also been used as reflectors. They arealso well known as optical components in the infrared (IR), where theyare used principally as transmissive polarizer elements.

[0023] Although it has not been demonstrated, some have postulatedpossible use of a wire grid polarizer in display applications in thevisible portion of the spectrum. For example, Grinberg (U.S. Pat. No.4,688,897) suggested that a wire grid polarizer serve as both areflector and an electrode (but not simultaneously as an analyzer) for aliquid crystal display.

[0024] Others have posed the possible use of a wire grid polarizer inplace of a dichroic polarizer to improve the efficiency of virtual imagedisplays (see U.S. Pat. No. 5,383,053). The need for contrast orextinction in the grid polarizer, however, is explicitly dismissed, andthe grid is basically used as a polarization sensitive beam steeringdevice. It does not serve the purpose of either an analyzer, or apolarizer, in the U.S. Pat. No. 5,383,053 patent. It is also clear fromthe text that a broadband polarizing cube beam splitter would haveserved the purpose as well, if one had been available. This technology,however, is dismissed as being too restricted in acceptance angle toeven be functional, as well as prohibitively expensive.

[0025] Another patent (U.S. Pat. No. 4,679,910) describes the use of agrid polarizer in an imaging system designed for the testing of IRcameras and other IR instruments. In this case, the application requiresa beam splitter for the long wavelength infra-red, in which case a gridpolarizer is the only practical solution. The patent does not suggestutility for the visible range or even mention the need for a largeangular aperture. Neither does it address the need for efficientconversion of light into a viewable image, nor the need for broadbandperformance.

[0026] Other patents also exist for wire-grid polarizers in the infraredportion of the spectrum (U.S. Pat. Nos. 4,514,479, 4,743,093; and5,177,635, for example). Except for the exceptions just cited, theemphasis is solely on the transmission performance of the polarizer inthe IR spectrum.

[0027] These references demonstrate that it has been known for manyyears that wire-grid arrays can function generally as polarizers.Nevertheless, they apparently have not been proposed and developed forimage projection systems. One possible reason that wire grid polarizershave not been applied in the visible spectrum is the difficulty ofmanufacture. U.S. Pat. No. 4,514,479 teaches a method of holographicexposure of photoresist and subsequent etching in an ion mill to make awire grid polarizer for the near infrared region; in U.S. Pat. No.5,122,907, small, elongated ellipsoids of metal are embedded in atransparent matrix that is subsequently stretched to align their longaxes of the metal ellipsoids to some degree. Although the transmittedbeam is polarized, the device does not reflect well. Furthermore, theellipsoid particles have not been made small enough to be useful in thevisible part of the electromagnetic spectrum. Accordingly, practicalapplications have been generally limited to the longer wavelengths ofthe IR spectrum.

[0028] Another prior art polarizer achieves much finer lines by grazingangle evaporative deposition (U.S. Pat. No. 4,456,515). Unfortunately,the lines have such small cross sections that the interaction with thevisible light is weak, and so the optical efficiency is too poor for usein the production of images. As in several of these prior art efforts,this device has wires with shapes and spacings that are largely random.Such randomness degrades performance because regions of closely spacedelements do not transmit well, and regions of widely spaced elementshave poor reflectance. The resulting degree of polarization (extinction)is less than maximal if either or both of these effects occur, as theysurely must if placement has some randomness to it.

[0029] For perfect (and near perfect) regularity, the mathematicsdeveloped for gratings apply well. Conversely, for random wires (even ifthey all have the same orientation) the theory of scattering providesthe best description. Scattering from a single cylindrical wire has beendescribed (H. C. Van de Hulst, Light Scattering by Small Particles,Dover, 1981). The current random-wire grids have wires embeddedthroughout the substrate. Not only are the positions of the wiressomewhat random, but the diameters are as well. It is clear that thephases of the scattered rays will be random, so the reflection will notbe strictly specular and the transmission will not retain high spacialor image fidelity. Such degradation of the light beam would prevent itsuse for transfer of well resolved, high-information density images.

[0030] Nothing in the prior art indicates or suggests that an orderedarray of wires can or should be made to operate over the entire visiblerange as a spathic PBS, at least at the angles required when it servesboth as a polarizer and analyzer. Indeed, the difficulty of making thenarrow, tall, evenly spaced wires that are required for such operationhas been generously noted (see Zeitner, et. al. Applied Optics, 38, 11pp. 2177-2181 (1999), and Schnabel, et. al., Optical Engineering 38,2pp. 220-226 (1999)). Therefore, it is not surprising that the prior artfor image projection similarly makes no suggestion for use of a spathicPBS as part of a display device.

[0031] Tamada and Matsumoto (U.S. Pat. No. 5,748,368) disclose a wiregrid polarizer that operates in both the infrared and a portion of thevisible spectrum; however, it is based on the concept that large, widelyspaced wires will create resonance and polarization at an unexpectedlyshort wavelength in the visible. Unfortunately, this device works wellonly over a narrow band of visible wavelengths, and not over the entirevisible spectrum. It is therefore not suitable for use in producingimages in full color. Accordingly, such a device is not practical forimage display because a polarizer must be substantially achromatic foran image projection system.

[0032] Another reason wire grid polarizers have been overlooked is thecommon and long standing belief that the performance of a typical wiregrid polarizer becomes degraded as the light beam's angle of incidencebecomes large (G.R. Bird and M. Parrish, Jr., “The Wire Grid as aNear-Infrared Polarizer,” J. Opt. Soc. Am., 50, pp. 886-891, (1960); theHandbook of Optics, Michael Bass, Volume II, p. 3-34, McGraw-Hill(1995)). There are no reports of designs that operate well for anglesbeyond 35° incidence in the visible portion of the spectrum. Nor hasanyone identified the important design factors that cause thislimitation of incidence angle. This perceived design limitation becomeseven greater when one realizes that a successful beam splitter willrequire suitable performance in both transmission and reflectionsimultaneously.

[0033] This important point deserves emphasis. The extant literature andpatent history for wire grid polarizers in the IR and the visiblespectra has almost entirely focused on their use as transmissionpolarizers, and not on reflective properties. Wire grid polarizers havebeen attempted and reported in the technical literature for decades, andhave become increasingly common since the 1960s. Despite the extensivework done in this field, there is very little, if any, detaileddiscussion of the production and use of wire grid polarizers asreflective polarizers, and nothing in the literature concerning theiruse as both transmissive and reflective polarizers simultaneously, aswould be necessary in a spathic polarizing beam splitter for use inimaging devices. From the lack of discussion in the literature, areasonable investigator would conclude that any potential use of wiregrid polarizers as broadband visible beam splitters is not apparent, orthat it was commonly understood by the technical community that theiruse in such an application was not practical.

[0034] Because the conventional polarizers described above were the onlyones available, it was impossible for Takanashi (U.S. Pat. No.5,239,322) to reduce his projection device to practice with anything butthe most meager results. No polarizer was available which supplied theperformance required for the Takanashi invention, namely, achromaticityover the visible part of the spectrum, wide angular acceptance, lowlosses in transmission and reflection of the desired lightpolarizations, and good extinction ratio.

[0035] There are several important features of an image display systemwhich require specialized performance of transmission and reflectionproperties. For a projector, the product of p-polarization transmissionand s-polarization reflection (R_(S)T_(P)) must be large if source lightis to be efficiently placed on the screen. On the other hand, for theresolution and contrast needed to achieve high information density onthe screen, it is important that the converse product (R_(P)T_(S)) bevery small (i.e. the transmission of s-polarized light multiplied by thereflection of p-polarized light must be small).

[0036] Another important feature is a wide acceptance angle. Theacceptance angle must be large if light gathering from the source, andhence the conversion efficiency, is maximized. It is desirable thatcones of light (either diverging or converging) with half-angles greaterthan 20° be accepted.

[0037] An important consequence of the ability to accept larger lightcones and work well at large angles is that the optical design of theimaging system is no longer restricted. Conventional light sources canbe then be used, bringing their advantages of low cost, cool operation,small size, and low weight. A wide range of angles makes it possible forthe designer to position the other optical elements in favorablepositions to improve the size and operation of the display.

[0038] Another important feature is size and weight. The conventionaltechnology requires the use of a glass cube. This cube imposes certainrequirements and penalties on the system. The requirements imposedinclude the need to deal with thermal loading of this large piece ofglass and the need for high quality materials without stressbirefringence, etc., which impose additional cost. In addition, theextra weight and bulk of the cube itself poses difficulties. Thus, it isdesirable that the beam splitter not occupy much volume and does notweigh very much.

[0039] Another important feature is robustness. Modern light sourcesgenerate very high thermal gradients in the polarizer immediately afterthe light is switched on. At best, this can induce thermal birefringencewhich causes cross talk between polarizations. What is more, the longduration of exposure to intense light causes some materials to changeproperties (typically yellowing from photo-oxidation). Thus, it isdesirable for the beam splitter to withstand high temperatures as wellas long periods of intense radiation from light sources.

[0040] Still another important feature is uniform extinction (orcontrast) performance of the beam splitter over the incident cone oflight. A McNeille-type thin film stack polarizer produces polarizedlight due to the difference in reflectivity of S-polarized light asopposed to P-polarized light. Since the definition of S and Ppolarization depends on the plane of incidence of the light ray, whichchanges orientation within the cone of light incident on the polarizer,a McNeille-type polarizer does not work equally well over the entirecone. This weakness in McNeille-type polarizers is well known. It mustbe addressed in projection system design by restricting the angular sizeof the cone of light, and by compensation elsewhere in the opticalsystem through the use of additional optical components. Thisfundamental weakness of McNeille prisms raises the costs andcomplexities of current projection systems, and limits systemperformance through restrictions on the f-number or optical efficiencyof the beam splitter.

[0041] Other important features include ease of alignment. Productioncosts and maintenance are both directly affected by assembly criteria.These costs can be significantly reduced with components which do notrequire low tolerance alignments.

[0042] The prior patent (U.S. Pat. No. 6,234,634) advantageously teachesthe use of a wire grid polarizer as the PBS for both polarizing andanalyzing in an image projection system. However, the wire gridpolarizer itself presents various challenges. For example, the wire gridcan be fragile or susceptible to damage in environments with highhumidity, significant air pollution, or other conditions. Thus, it isdesirable to protect the wire grid. Because wire grid polarizers arewavelength sensitive optical devices, imbedding the polarizer in amaterial or medium with an index of refraction greater than one willalways change the performance of the polarizer over that available inair for the same structure. Typically, this change renders the polarizerless suitable for the intended application. Imbedding the polarizer,however, provides other optical advantages. For example, imbedding thepolarizer may provide other beneficial optical properties, and mayprotect the polarizer, although the performance of the polarizer itself,or polarization, may be detrimentally effected. Therefore, it isdesirable to obtain the optimum performance of such an imbeddedwire-grid polarizer.

[0043] Wire grids are typically disposed on an outer surface of asubstrate, such as glass. Some wire grids have been totally encased inthe substrate material, or glass. For example, U.S. Pat. No. 2,224,214,issued Dec. 10, 1940, to Brown, discloses forming a polarizer by meltinga powdered glass packed around wires, and then stretching the glass andwires. Similarly, U.S. Pat. No. 4,289,381, issued Sep. 15, 1981, toGarvin et al., discloses forming a polarizer by depositing a layer ofmetallization on a substrate to form the grid, and then depositingsubstrate material over the grid. In either case, the wires or grid aresurrounded by the same material as the substrate. As stated above, suchencasement of the wires or grids detrimentally effects the opticalperformance of the grid.

[0044] U.S. Pat. No. 5,748,368, issued May 5, 1998, to Tamada et al.,discloses a narrow bandwidth polarizer with a grid disposed on asubstrate, and a wedge glass plate disposed over the grid. A matchingoil is also applied over the elements which is matched to have the samerefractive index as the substrate. Thus, the grid is essentially encasedin the substrate or glass because the matching oil has the samerefractive index. Again, such encasement of the grid detrimentallyeffects the optical performance of the gird.

[0045] The key factor that determines the performance of a wire gridpolarizer is the relationship between the center-to-center spacing, orperiod, of the parallel grid elements and the wavelength of the incidentradiation. If the grid spacing or period is long compared to thewavelength, the grid functions as a diffraction grating, rather than asa polarizer, and diffracts both polarizations (not necessarily withequal efficiency) according to well-known principles. When the gridspacing or period is much shorter than the wavelength, the gridfunctions as a polarizer that reflects electromagnetic radiationpolarized parallel to the grid elements, and transmits radiation of theorthogonal polarization.

[0046] The transition region, where the grid period is in the range ofroughly one-half of the wavelength to twice the wavelength, ischaracterized by abrupt changes in the transmission and reflectioncharacteristics of the grid. In particular, an abrupt increase inreflectivity, and corresponding decrease in transmission, for lightpolarized orthogonal to the grid elements will occur at one or morespecific wavelengths at any given angle of incidence. These effects werefirst reported by Wood in 1902 (Philosophical Magazine, September 1902),and are often referred to as “Wood's Anomalies”. Subsequently, Rayleighanalyzed Wood's data and had the insight that the anomalies occur atcombinations of wavelength and angle where a higher diffraction orderemerges (Philosophical Magazine, vol. 14(79), pp. 60-65, July 1907).Rayleigh developed an equation to predict the location of the anomalies(which are also commonly referred to in the literature as “RayleighResonances”).

[0047] The effect of the angular dependence is to shift the transmissionregion to larger wavelengths as the angle increases. This is importantwhen the polarizer is intended for use as a polarizing beam splitter orpolarizing turning mirror because such uses require high angles ofincidence.

[0048] A wire grid polarizer is comprised of a multiplicity of parallelconductive electrodes supported by a substrate. Such a device ischaracterized by the pitch or period of the conductors; the width of theindividual conductors; and the thickness of the conductors. A beam oflight produced by a light source is incident on the polarizer at anangle Θ from normal, with the plane of incidence orthogonal to theconductive elements. The wire grid polarizer divides this beam into aspecularly reflected component, and a non-diffracted, transmittedcomponent. For wavelengths shorter than the longest resonancewavelength, there will also be at least one higher-order diffractedcomponent. Using the normal definitions for S and P polarization, thelight with S polarization has the polarization vector orthogonal to theplane of incidence, and thus parallel to the conductive elements.Conversely, light with P polarization has the polarization vectorparallel to the plane of incidence and thus orthogonal to the conductiveelements.

[0049] In general, a wire grid polarizer will reflect light with itselectric field vector parallel to the wires of the grid, and transmitlight with its electric field vector perpendicular to the wires of thegrid, but the plane of incidence may or may not be perpendicular to thewires of the grid as discussed here.

[0050] Ideally, the wire grid polarizer will function as a perfectmirror for one polarization of light, such as the S polarized light, andwill be perfectly transparent for the other polarization, such as the Ppolarized light. In practice, however, even the most reflective metalsused as mirrors absorb some fraction of the incident light and reflectonly 90% to 95%, and plain glass does not transmit 100% of the incidentlight due to surface reflections.

[0051] Applicants' prior patent (U.S. Pate. No. 6,122,103) showstransmission and reflection of a wire grid polarizer with two resonanceswhich only affect significantly the polarizer characteristics for Ppolarization. For incident light polarized in the S direction, thereflectivity of the polarizer approaches the ideal. The reflectionefficiency for S polarization is greater than 90% over the visiblespectrum from 0.4 μm to 0.7 μm. Over this wavelength band, less than2.5% of the S polarized light is transmitted, with the balance beingabsorbed. Except for the small transmitted component, thecharacteristics of the wire grid polarizer for S polarization are verysimilar to those of a continuous aluminum mirror.

[0052] For P polarization, and high angle of incidence, the transmissionand reflection efficiencies of the wire grid are affected by theresonance effect at wavelengths below about 0.5 μm. At wavelengthslonger than 0.5 μm, the wire grid structure acts as a lossy dielectriclayer for P polarized light. The losses in this layer and thereflections from the surfaces combine to limit the transmission for Ppolarized light.

[0053] Applicants' prior patent (U.S. Pat. No. 6,122,103) also shows thecalculated performance of a different type of prior-art wire girdpolarizer, as described by Tamada in U.S. Pat. No. 5,748,368. As statedabove, an index matching fluid has been used between two substrates suchthat the grid is surrounded by a medium of constant refractive index.This wire grid structure exhibits a single resonance at a wavelengthabout 0.52 μm. There is a narrow wavelength region, from about 0.58 to0.62 μm, where the reflectivity for P polarization is very nearly zero.U.S. Pat. No. 5,748,368 describes a wire grid polarizer that takesadvantage of this effect to implement a narrow bandwidth wire girdpolarizer with high extinction ratio. The examples given in the Tamadapatent specification used a grid period of 550 nm, and produced aresonance wavelength from 800 to 950 nm depending on the grid thickness,conductor width and shape, and the angle of incidence. The resonanceeffect that Tamada exploits is different from the resonance whoseposition is described above. While the two resonances may be coincident,they do not have to be. Tamada exploits this second resonance.Furthermore, there are thin film interference effects which may comeinto play. The bandwidth of the polarizer, where the reflectivity forthe orthogonal-polarized light is less than a few percent, is typically5% of the center wavelength. While this type of narrow band polarizermay have some applications, many visible-light systems, such as liquidcrystal displays, require polarizing optical elements with uniformcharacteristics over the visible-spectrum wavelengths from 400 nm to 700nm.

[0054] A necessary requirement for a wide band polarizer is that thelongest wavelength resonance point must either be suppressed or shiftedto a wavelength shorter than the intended spectrum of use. Thewavelength of the longest-wavelength resonance point can be reduced inthree ways. First, the grid period can be reduced. However, reducing thegrid period increases the difficulty of fabricating the grid structure,particularly since the thickness of the grid elements must be maintainedto ensure adequate reflectivity of the reflected polarization. Second,the incidence angle can be constrained to near-normal incidence.However, constraining the incidence angle would greatly reduce theutility of the polarizer device, and preclude its use in applicationssuch as projection liquid crystal displays where a wide angularbandwidth centered on 45 degrees is desired. Third, the refractive indexof the substrate could be lowered. However, the only cost-effectivesubstrates available for volume manufacture of a polarizer device areseveral varieties of thin sheet glass, such as Corning type 1737F orSchott type AF45, all of which have a refractive index which variesbetween 1.5 and 1.53 over the visible spectrum.

[0055] As stated above, the wire grid polarizer can include amultiplicity of parallel conductive electrodes supported by a substrate.The substrate itself, however, can have certain optical consequencesthat can limit the utility of a wire grid polarizer used in such animage display described above. For example, the substrate can causeaberrations of astigmatism and coma if a non-collimated beam of lightpasses through the substrate tilted at an angle. One reason cubepolarizing beam splitters are sometimes used is because light enterssuch cube polarizers with the optic axis normal to the cube surface,thus minimizing these aberrations.

[0056] Light striking the substrate at other than normal incidence cansuffer from a lateral shift in position along the sloping direction ofthe substrate. Consequently, a diverging light cone striking thesubstrate suffers astigmatic aberration and coma causing the otherwiseround area of the beam to become elongated in one direction. This,combined with chromatic aberration (color separation) as polychromaticlight disperses through the tilted substrate, causes unacceptabledistortion in high quality imaging optical systems. These aberrationsoccur regardless of the flatness of the substrate. Therefore, flat platetransmissive optics cannot be used in imaging applications unless theaberrations are corrected or rendered negligible.

SUMMARY OF THE INVENTION

[0057] It has been recognized that it would be advantageous to developan image projection system capable of providing bright images and goodimage contrast, and which is inexpensive. It also has been recognizedthat it would be advantageous to develop an image projection system witha polarizing beam splitter that reduces aberrations of astigmatism andcoma, and/or that produces a transmitted or reflected beam with reducedgeometric distortions. It also has been recognized that it would beadvantageous to develop an image projection system with a polarizingbeam splitter which is protected against environmental degradation andother sources of damage while reducing detrimental effects of theprotection on the performance of the beam splitter.

[0058] It also has been recognized that it would be advantageous todevelop an image projection system with a polarizing beam splittercapable of utilizing divergent light (or having a smaller F/#), capableof efficient use of light energy or with high conversion efficiency, andwhich is durable. It also has been recognized that it would beadvantageous to develop an image projection system with a polarizingbeam splitter having a high extinction ratio, uniform response over thevisible spectrum, good color fidelity, that is spathic, robust andcapable of resisting thermal gradients.

[0059] It also has been recognized that it would be advantageous todevelop an image projection system with a polarizing beam splittercapable of being positioned at substantially any incidence angle so thatsignificant design constraints are not imposed on the image projectionsystem, but substantial design flexibility is permitted. It also hasbeen recognized that it would be advantageous to develop an imageprojection system with a polarizing beam splitter which efficientlytransmits p-polarized light and reflects s-polarized light across allangles in the entire cone of incident light. It also has been recognizedthat it would be advantageous to develop an image projection system witha polarizing beam splitter which is light-weight and compact. It alsohas been recognized that it would be advantageous to develop an imageprojection system with a polarizing beam splitter which is easy toalign.

[0060] The invention provides an image projection system with apolarizing beam splitter which advantageously is a wire grid polarizer.The wire grid polarizing beam splitter has a generally parallelarrangement of thin, elongated elements. The arrangement is configured,and the elements are sized, to interact with electromagnetic waves ofthe source light beam to generally transmit one polarization of lightthrough the elements, and reflect the other polarization from theelements. Light having a polarization oriented perpendicular to a planethat includes at least one of the elements and the direction of theincident light beam is transmitted, and defines a transmitted beam. Theopposite polarization, or light having a polarization oriented parallelwith the plane that includes at least one of the elements and thedirection of the incident light beam, is reflected, and defines areflected beam.

[0061] The system includes a light source for producing a visible lightbeam. The polarizing beam splitter is located proximal to the lightsource in the light beam. The system also includes a reflective liquidcrystal array. The array may be located proximal to the polarizing beamsplitter in either the reflected or transmitted beam. The arraymodulates the polarization of the beam, and creates a modulated beam.The array is oriented to direct the modulated beam back to the beamsplitter. The arrangement of elements of the beam splitter interactswith electromagnetic waves of the modulated beam to again generallytransmit one polarization and reflect the other polarization. Thus, thereflected portion of the modulated beam defines a second reflected beam,while the transmitted portion defines a second transmitted beam. Thearray alters the polarization of the beam to encode image information onthe modulated beam. The beam splitter separates the modulatedpolarization from the unmodulated beam, thus making the image visible ona screen.

[0062] A screen is disposed in either the second reflected or secondtransmitted beam. If the array is disposed in the reflected beam, thenthe screen is disposed in the second transmitted beam. If the array isdisposed in the transmitted beam, then the screen is disposed in thesecond reflected beam.

[0063] Unlike the bulky, heavy beam splitters of the prior art, the beamsplitter of the present invention is a generally planar sheet. The beamsplitter is also efficient, thus providing greater luminous efficacy ofthe system.

[0064] In accordance with one aspect of the present invention, the beamsplitter advantageously includes an embedded wire grid polarizer with anarray of parallel, elongated, spaced-apart elements sandwiched betweenfirst and second layers. The elements form a plurality of gaps betweenthe elements, and the gaps advantageously provide a refractive indexless than the refractive index of the first or second layer. Preferably,the gaps include air or have a vacuum.

[0065] In accordance with another aspect of the present invention, theelements of the wire grid polarizer can be disposed on a substrate.Preferably, the substrate is very thin, or has a thickness less thanapproximately 5 millimeters, to reduce astigmatism, coma, and/orchromatic aberrations. In addition, the wire grid polarizer andsubstrate preferably transmits a transmitted beam with reduced geometricdistortions, preferably less than approximately 3 standard wavelengthsper inch.

[0066] In accordance with another aspect of the present invention, thesubstrate preferably has a surface with a flatness less thanapproximately 3 standard wavelengths deviation per inch to reducedistortions in the reflected beam.

[0067] In accordance with another aspect of the present invention, thebeam splitter is capable of being oriented with respect to the lightbeam and the modulated beam at incidence angles between approximately 0to 80 degrees.

[0068] In accordance with another aspect of the present invention, thelight beam has a useful divergent cone with a half angle betweenapproximately 10 and 25°. The beam splitter is used at a small F-number,preferably between approximately 1.2 and 2.5.

[0069] In accordance with another aspect of the present invention, thebeam splitter has a conversion efficiency of at least 50% defined by theproduct of the s-polarization reflected light and the p-polarizationtransmitted light (R_(S)T_(P)). In addition, the s-polarizationtransmitted light and the p-polarization reflected light are both lessthan 5%. Furthermore, the percentage of reflected light and thepercentage of the transmitted light of the modulated beam is greaterthan approximately 67%.

[0070] In accordance with another aspect of the present invention, thesystem may include a pre-polarizer disposed between the light source andthe beam splitter, and/or a post-polarizer disposed between the beamsplitter and the screen.

[0071] Additional features and advantages of the invention will beapparent from the detailed description which follows, taken inconjunction with the accompanying drawings, which together illustrate,by way of example, features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0072]FIG. 1a is a schematic view of the general operation of apreferred embodiment of an image projection system of the presentinvention using a wire grid polarizing beam splitter of the presentinvention.

[0073]FIGS. 1b and 1 c are schematic views of the image projectionsystem of the present invention in different configurations.

[0074]FIG. 2a is a graphical plot showing the relationship betweenwavelength and transmittance for both S and P polarizations of apreferred embodiment of the wire grid polarizing beam splitter of thepresent invention.

[0075]FIG. 2b is a graphical plot showing the relationship betweenwavelength and reflectance for both S and P polarizations of a preferredembodiment of the wire grid polarizing beam splitter of the presentinvention.

[0076]FIG. 2c is a graphical plot showing the relationship betweenwavelength, efficiency and transmission extinction of a preferredembodiment of the wire grid polarizing beam splitter of the presentinvention.

[0077]FIG. 3 is a graphical plot showing the performance of thepreferred embodiment of the wire grid polarizing beam splitter of thepresent invention as a function of the incident angle.

[0078]FIG. 4a is a graphical plot showing the theoretical throughputperformance of an alternative embodiment of the wire grid polarizingbeam splitter of the present invention.

[0079]FIG. 4b is a graphical plot showing the theoretical extinctionperformance of an alternative embodiment of the wire grid polarizingbeam splitter of the present invention.

[0080]FIG. 4c is a graphical plot showing the theoretical extinctionperformance of an alternative embodiment of the wire grid polarizingbeam splitter of the present invention.

[0081]FIG. 5a is a schematic view of the general operation of analternative embodiment of an image projection system of the presentinvention.

[0082]FIGS. 5b and 5 c are schematic views of the image projectionsystem of the present invention in different configurations.

[0083]FIG. 6 is a schematic view of the general operation of analternative embodiment of an image projection system of the presentinvention.

[0084]FIG. 7 is a perspective view of the wire grid polarizing beamsplitter of the present invention.

[0085]FIG. 8 is a cross sectional side view of the wire grid polarizingbeam splitter of the present invention.

[0086]FIG. 9 is a cross-sectional view of an embedded wire gridpolarizer of the present invention.

DETAILED DESCRIPTION

[0087] For the purposes of promoting an understanding of the principlesof the invention, reference will now be made to the exemplaryembodiments illustrated in the drawings, and specific language will beused to describe the same. It will nevertheless be understood that nolimitation of the scope of the invention is thereby intended. Anyalterations and further modifications of the inventive featuresillustrated herein, and any additional applications of the principles ofthe invention as illustrated herein, which would occur to one skilled inthe relevant art and having possession of this disclosure, are to beconsidered within the scope of the invention.

[0088] As illustrated in FIG. la, a display optical train of an imageprojection system of the present invention, indicated generally at 10,is shown. The image projection system 10 advantageously has a wire gridpolarizer as the beam splitter, indicated generally at 14. The wire gridpolarizing beam splitter 14 (WGP-PBS) efficiently reflects light of onepolarization from a source 20 to a reflective liquid crystal array 26,and then efficiently transmits reflected light of the oppositepolarization to a display screen 25.

[0089] For adequate optical efficiency, the WGP-PBS 14 must have highreflectivity (R_(S)) of the desired polarization from the light source20, and it must have high transmissivity (T_(P)) of the oppositepolarization from the liquid crystals array 26. The conversionefficiency is proportional to the product of these two, R_(S)T_(P), sodeficiency in one factor can be compensated to some extent byimprovement in the other.

[0090] Examples of wire grid polarizing beam splitters 14 of the presentinvention advantageously show the following characteristics whichdemonstrate the advantage of using a WGP-PBS 14 of the present inventionas both the polarizer and analyzer in display devices for the visibleportion of the spectrum. Theoretical calculations of furtherimprovements indicate that even better polarizing beam splitters will beavailable.

[0091] Referring to FIGS. 2a and 2 b, the measured transmissivity andreflectivity, respectively, for both S and P polarizations of a WGP-PBSare shown. In FIG. 2c, the efficiency of the WGP-PBS is shown as theproduct of the transmissivity and reflectivity. In addition, theextinction is also shown in FIG. 2c. In FIGS. 2a-2 c, the WGP-PBS isoriented to reflect the s-polarization and transmit the p-polarizationat incident angles of 30°, 45° and 60°. For an image projection system,such as a projector, the product of the reflected s-polarization andtransmitted p-polarization (R_(S)T_(P)) must be large if source light isto be efficiently placed on the screen. On the other hand, for theresolution needed to achieve high information density on the screen, itis important that the converse product (R_(P)T_(S)) be very small (i.e.the transmission of s-polarized light multiplied by the reflection ofp-polarized light must be small). It is clear from the figures that thewire grid polarizing beam splitter of the present invention meets thesestandards over the entire spectrum without degradation by Rayleighresonance or other phenomena.

[0092] Another important feature is a wide acceptance angle. This mustbe large if light gathering from the source, and hence the conversionefficiency, is maximized. Referring to FIG. 3, the performance of thewire grid polarizing beam splitter of the present invention is shown forvarious portions of the light cone centered around the optical axiswhich is inclined at 45°. In FIG. 3, the first referenced angle is theangle in the plane of incidence while the second referenced angle is theangle in the plane perpendicular to the plane of incidence. It is clearthat the WGP-PBS of the present invention is able to accept cones oflight (either diverging or converging) with half-angles betweenapproximately 10 and 25°.

[0093] Referring to FIGS. 4a-4 c, theoretical calculations for analternative embodiment of a wire grid polarizing beam splitter indicatethat significantly larger light cones and/or other enhancements will bepossible. FIGS. 4a and 4 b show the theoretical throughput andextinction, respectively, of a wire grid polarizing beam splitter with aperiod p reduced to 130 nm. In addition, the grid height or thickness is130 nm; the line-spacing ratio is 0.48; the substrate groove depth is 50nm; and the substrate is BK7 glass. It should be noted in FIG. 4a thatthe throughput is grouped much more closely than the throughput shown inFIG. 2c. Therefore, performance can be improved by reducing the periodp. It should be noted in FIG. 4b that the extinction is significantlyincreased in comparison to FIG. 2c.

[0094]FIG. 4c shows the theoretical extinction of another alternativeembodiment of the wire grid polarizing beam splitter with the period pfurther reduced. The wavelength is 420 nm and the incidence angle is30°. It should be noted that the extinction increases markedly as theperiod p is reduced.

[0095] As indicated above, an important consequence of the ability toaccept larger light cones with a WGP-PBS that will work well at largeangles is that the PBS no longer restricts the optical design of theimaging system. Thus, conventional light sources can be used, with theadvantage of their low cost, cooler operation, small size, and lowweight. The wide range of angles over which the WGP-PBS works well makesit possible for the designer to position the other optical elements infavorable positions to improve the size and operation of the display.Referring to FIGS. 1b and 1 c, the design flexibility provided by thewide range of angles of the PBS of the present invention isdemonstrated. As shown in FIG. 1b, the light source 20 and array 26 maybe positioned closer together, with both having a relatively smallincident angle with respect to the PBS 14. Such a configuration isadvantageous for a compact design of the components of the system 10.Alternatively, as shown in FIG. 1c, the light source 20 and array 26 maybe positioned farther apart, with both having a relatively largeincident angle. In should be noted that in either case, the incidenceangles vary greatly from the 45 degree angle typically required bytraditional beam splitters.

[0096] Yet other features of wire grids provide advantages for displayunits. The conventional technology requires the use of a glass cube.This cube imposes certain requirements and penalties on the system. Therequirements imposed include the need to deal with thermal loading ofthis large piece of glass, the need for high quality materials withoutstress birefringence, etc., which impose additional cost, and the extraweight and bulk of the cube itself. The WGP-PBS of the present inventionadvantageously is a divided or patterned thin film that does not occupymuch volume and does not weigh very much. It can even be integrated withor incorporated into other optical elements such as color filters, tofurther reduce part count, weight, and volume of the projection system.

[0097] The WGP-PBS of the present invention is also very robust. Modernlight sources generate very high thermal gradients in the polarizerimmediately after the light is switched on. At best, this can inducethermal and stress birefringence which causes cross talk betweenpolarizations. At worst, it can delaminate multilayer polarizers orcause the cemented interface in a cube beam splitter to separate. Whatis more, the long duration of exposure to intense light causes somematerials to change properties (typically yellowing fromphoto-oxidation). However, wire grid polarizing beam splitters are madeof chemically inert metal that is well attached to glass or othersubstrate materials. They have been shown to withstand high temperaturesas well as long periods of intense radiation from light sources.

[0098] The WGP-PBS of the present invention also is easy to align. It isa single part that needs to be adjusted to direct the source beam ontothe liquid crystal array. This is the same simple procedure that wouldbe used for a flat mirror. There is another adjustment parameter,namely, the angular rotation about the normal to the WGP surface. Thisdetermines the orientation of polarization in the light beam. Thisadjustment is not critical because the WGP functions as its own analyzerand cannot be out of alignment in this sense. If there are otherpolarizing elements in the optical train, the WGP-PBS should be orientedwith respect to their polarization, but slight misalignment is notcritical because: according to Malus' law, angular variation makes verylittle difference in the intensity transmitted by polarizers if theirpolarization axes are close to being parallel (or perpendicular).

[0099] In order to be competitive with conventional polarizers, theproduct R_(S)T_(P) must be above approximately 50%. This represents alower estimate which would only be practical if the WGP-PBS was able togather significantly more light from the light source than theconventional polarizing beam splitters. The estimate of 50% comes froman assumption that the best conventional beam splitter, a modernMacNeille cube beam splitter, can deliver an f/4 of about f/2.5 at best.An optical system which was twice as fast, or capable of gathering twiceas much light, would then have an f/# of 1/{square root}2 of this value,or about f/1.8, which is certainly a reasonable f/# in optical imageprojection systems. A system which is twice as fast, and thereforecapable of gathering twice the light from the source, wouldapproximately compensate for the factor of 2 decrease in the R_(S)T_(P)product over the conventional cube beam splitter, resulting in anequivalent projection system performance. In fact, since a WGP-PBS canpotentially be used down below f/1.2 (a factor of four increase) thisseemingly low limit can still produce very bright images. Of course, anR_(S)T_(P) product which is over this minimum value will provide evenbetter performance.

[0100] Another important performance factor is contrast in the image, asdefined by the ratio of intensities of light to dark pixels. One of thesignificant advantages of the WGP-PBS is the improved contrast overcompound incident angles in comparison to the prior art cube beamsplitter such as a McNeille prism. The physics of the McNeille prismpolarizes light by taking advantage of the difference in reflectivity ofS vs. P polarization at certain angles. Because S and P polarization aredefined with respect to the plane of incidence, the effective S and Ppolarization for a particular ray in a cone of light rotates withrespect to the ray along the optical axis as various rays within thecone of light are considered. The consequence of this behavior is thewell-known compound angle problem in which the extinction of thepolarizer is significantly reduced for certain ranges of angles withinthe cone of light passing through the polarizing beam splitter,significantly reducing the average contrast over the cone.

[0101] The WGP-PBS, on the other hand, employs a different physicalmechanism to accomplish the polarization of light which largely avoidsthis problem. This difference in behavior is due to the fact that thepolarization is caused by the wire grids in the beam splitter which havethe same orientation in space regardless of the plane of incidence forany particular ray in the cone of light. Therefore, even though theplane of incidence for any particular ray is the same when incident on aMcNeille prism or a WGP, the polarization effect is only dependent onthe plane of incidence in the case of the McNeille prism, meaning thecompound angle performance of the WGP is much improved over thatprovided by the cube beam splitter.

[0102] The fact that the function of the WGP-PBS is independent of theplane of incidence means that the WGP-PBS can actually be used with thewires or elements oriented in any direction. The preferred embodiment ofthe invention has the elements oriented parallel to the axis aroundwhich the polarizer is tilted so that the light strikes the WGP-PBS atan angle. This particular orientation is preferred because it causes thepolarization effects of the surface reflections from the substrate to beadditive to the polarization effects from the grid. It is possible,however, to produce a WGP-PBS which functions to reflect theP-polarization and transmit the S-polarization (which is exactlyopposite what has been generally described herein) over certain rangesof incident angles by rotating the grid elements so they areperpendicular to the tilt axis of the WGP-PBS. Similarly, the gridelements can be placed at an arbitrary angle to the tilt axis to obtaina WGP-PBS which functions to transmit and reflect light withpolarizations aligned with the projection of this arbitrary angle ontothe wavefront in the light beam. It is therefore clear that WGP-PBSwhich reflect the P-polarization and transmit the S-polarization, orwhich reflect and transmit light with polarization oriented at arbitraryangles are included within this invention.

[0103] The compound angle performance advantage of the WGP-PBS providesan inherently more uniform contrast over the entire light cone, and isone of the reasons the WGP is suitable for very small f-numbers. But, ofcourse, it is not the only factor affecting the image contrast. Theimage contrast is governed to a large extent by low leakage of theundesired polarization, but in this case the product T_(S)R_(P) is notthe important parameter, because the image generating array which liesin sequence after the first encounter with the beam splitter but beforethe second also takes part in the production of the image contrast.Therefore, the final system contrast will depend on the light valveperformance as well as the polarizer extinction. However, lower boundson the required beam splitter performance can be determined with theassumption that the light valve performance is sufficient enough that itcan be assumed to have an essentially infinite contrast. In this case,the system contrast will depend entirely on the beam splitterperformance.

[0104] Referring to FIG. la, there are two different functions fulfilledby the beam splitter 14. The first is the preparation of the polarizedlight before it strikes the liquid crystal array 26 or other suitableimage generation device. The requirement here is that the light besufficiently well polarized that any variations in the polarization ofthe light beam created by the light valve can be adequately detected oranalyzed such that the final image will meet the desired level ofperformance. Similarly, the beam splitter 14 must have sufficientperformance to analyze light which is directed by the light valve backto the beam splitter so that the desired system contrast performance isachieved.

[0105] These lower bounds can be determined fairly easily. For reasonsof utility and image quality, it is doubtful that an image with acontrast of less than 10:1 (bright pixel to adjacent dark pixel) wouldhave much utility. Such a display would not be useful for dense text,for example. If a minimum display system contrast of 10:1 is assumed,then an incident beam of light is required which has at least 10 timesthe light of the desired polarization state over that of the undesiredpolarization state. In terms of polarizer performance, this would bedescribed as having an extinction of 10:1 or of simply 10.

[0106] The second encounter with the beam splitter 14 which is going toanalyze the image, must be able to pass the light of the rightpolarization state, while eliminating most of the light of the undesiredstate. Again, assuming from above a light beam with an image encoded inthe polarization state, and that this light beam has the 10:1 ratioassumed, then a beam splitter is desired which preserves this 10:1 ratioto meet the goal of a system contrast of 10:1. In other words, it isdesired to reduce the light of the undesired polarization by a factor of10 over that of the right polarization. This again leads to a minimumextinction performance of 10:1 for the analysis function of the beamsplitter.

[0107] Clearly, higher system contrast will occur if either or both ofthe polarizer and analyzer functions of the beam splitter have a higherextinction performance. It is also clear that it is not required thatthe performance in both the analyzer function and the polarizer functionof the beam splitter be matched for a image projection system to performadequately. An upper bound on the polarizer and analyzer performance ofthe beam splitter is more difficult to determine, but it is clear thatextinctions in excess of approximately 20,000 are not needed in thisapplication. A good quality movie projection system as found in aquality theater does not typically have an image contrast over about1000, and it is doubtful that the human eye can reliably discriminatebetween an image with a contrast in the range of several thousand andone with a contrast over 10,000. Given a need to produce an image with acontrast of several thousand, and assuming that the light valves capableof this feat exist, an upper bound on the beam splitter extinction inthe range of 10,000-20,000 would be sufficient.

[0108] The above delineation of the minimum and maximum bounds on thewire grid beam splitter is instructive, but as is clear from thedemonstrated and theoretical performance of a wire grid beam splitter asshown above, much better than this can be achieved. In accordance withthis information, the preferred embodiment has R_(S)T_(P)≧65%, and R_(P)or T_(S) or both are ≧67%, as shown in FIGS. 2a-2 c. The preferredembodiment would also employ the wire grid polarizing beam splitter inthe mode where the reflected beam is directed to the image generatingarray, with the array directing the light back to the beam splitter suchthat it passes through, or is transmitted through, the beam splitter.This preferred embodiment is shown in FIG. 1a.

[0109] Alternatively, as shown in the image display system 60 of FIG.5a, the wire grid polarizing beam splitter 14 may efficiently transmitlight of one polarization from the source 20 to the reflective liquidcrystal array 26, and then efficiently reflect the reflected light ofthe opposite polarization to the display screen 25. The secondembodiment of the image projection system 60 is similar to that of thepreferred embodiment shown in FIG. 1a, with the exception that the beamsplitter 14 would be employed in a manner in which the source beam oflight is transmitted or passed through the beam splitter 14 and directedat the image generating array 26, then is reflected back to the beamsplitter 14 where it is reflected by the beam splitter and analyzedbefore being displayed on the screen 25.

[0110] Again, referring to FIGS. 5b and 5 c, the design flexibilityprovided by the wide range of angles of the PBS of the present inventionis demonstrated. As shown in FIG. 5b, the array 26 and screen 25 may bepositioned closer together, with both having a relatively small incidentangle with respect to the PBS 14. Alternatively, as shown in FIG. 5c,the array 26 and screen 25 may be positioned farther apart, with bothhaving a relatively large incident angle.

[0111] As shown in FIG. 6, a third embodiment of image projection system80 provides an alternative system design which may assist in achieving adesired level of system performance. This third embodiment would includeone or more additional transmissive or reflective polarizers which workin series with the wire grid polarizing beam splitter to increase theextinction of either or both of the polarizing and analyzing functionsto achieve the necessary system contrast performance. Another reason foradditional polarizers would be the implementation of a polarizationrecovery scheme to increase the system efficiency. A pre-polarizer 82 isdisposed in the source light beam between the light source 20 and theWGP-PBS 14. A post-polarizer or clean-up polarizer 84 is disposed in themodulated beam, or the beam reflected from the array 26, between thearray 26 and the screen 25, or between the WGP-PBS 14 and the screen 25.The third embodiment would still realize the advantages of the wire gridbeam splitter's larger light cone, durability, and the other advantagesdiscussed above.

[0112] As shown in the figures, the image display system may alsoutilize light gathering optics 90 and projection optics 92.

[0113] Referring to FIGS. 7 and 8, the wire grid polarizing beamsplitter 14 of the present invention is shown in greater detail. Thepolarizing beam splitter is further discussed in greater detail inco-pending U.S. application Ser. No. 09/390,833, filed Sep. 7, 1999,entitled “Polarizing Beam Splitter”, which is herein incorporated byreference.

[0114] As described in the co-pending application, the polarizing beamsplitter 14 has a grid 30, or an array of parallel, conductive elements,disposed on a substrate 40. The source light beam 130 produced by thelight source 20 is incident on the polarizing beam splitter 14 with theoptical axis at an angle Θ from normal, with the plane of incidencepreferably orthogonal to the conductive elements. An alternativeembodiment would place the plane of incidence at an angle 73 to theplane of conductive elements, with Θ approximately 45°. Still anotheralternative embodiment would place the plane of incidence parallel tothe conductive elements. The polarizing beam splitter 14 divides thisbeam 130 into a specularly reflected component 140, and a transmittedcomponent 150. Using the standard definitions for S and P polarization,the light with S polarization has the polarization vector orthogonal tothe plane of incidence, and thus parallel to the conductive elements.Conversely, light with P polarization has the polarization vectorparallel to the plane of incidence and thus orthogonal to the conductiveelements.

[0115] Ideally, the polarizing beam splitter 14 will function as aperfect mirror for the S polarized light, and will be perfectlytransparent for the P polarized light. In practice, however, even themost reflective metals used as mirrors absorb some fraction of theincident light, and thus the WGP will reflect only 90% to 95%, and plainglass does not transmit 100% of the incident light due to surfacereflections.

[0116] The key physical parameters of the wire grid beam splitter 14which must be optimized as a group in order to achieve the level ofperformance required include: the period p of the wire grid 30, theheight or thickness t of the grid elements 30, the width w of the gridselements 30, and the slope of the grid elements sides. It will be notedin examining FIG. 8 that the general cross-section of the grid elements30 is trapezoidal or rectangular in nature. This general shape is also anecessary feature of the polarizing beam splitter 14 of the preferredembodiment, but allowance is made for the natural small variations dueto manufacturing processes, such as the rounding of corners 50, andfillets 54, at the base of the grid elements 30.

[0117] It should also be noted that the period p of the wire grid 30must be regular in order to achieve the specular reflection performancerequired to meet the imaging fidelity requirements of the beam splitter14. While it is obviously better to have the grid 30 completely regularand uniform, some applications may have relaxed requirements in whichthis is not as critical. However, it is believed that a variation inperiod p of less than 10% across a meaningful dimension in the image(such as the size of a single character in a textual display, or a fewpixels in an image) is required to achieve the necessary performance.

[0118] Similarly, reasonable variations across the beam splitter 14 inthe other parameters described, such as the width w of the grid elements30, the grid element height t, the slopes of the sides, or even thecorner rounding 50, and the fillets 54, are also possible withoutmaterially affecting the display performance, especially if the beamsplitter 14 is not at an image plane in the optical system, as willoften be the case. These variations may even be visible in the finishedbeam splitter 14 as fringes, variations in transmission efficiency,reflection efficiency, color uniformity, etc. and still provide a usefulpart for specific applications in the projection imaging system.

[0119] The design goal which must be met by the optimization of theseparameters is to produce the best efficiency or throughput possible,while meeting the contrast requirements of the application. As statedabove, the minimum practical extinction required of the polarizing beamsplitter 14 is on the order of 10. It has been found that the minimumrequired throughput (R_(S)T_(P)) of the beam splitter 14 in order tohave a valuable product is approximately 50%, which means either or bothof R_(P) and T_(S) must be above about 67%. Of course, higherperformance in both the throughput and the extinction of the beamsplitter will be of value and provide a better product. In order tounderstand how these parameters affect the performance of the wire gridbeam splitter, it is necessary to examine the variation in performanceproduced by each parameter for an incident angle of 45°, and probablyother angles of interest.

[0120] The performance of the wire grid beam splitter 14 is a functionof the period p. The period p of the wire grid elements 30 must fallunder approximately 0.21 μm to produce a beam splitter 14 which hasreasonable performance throughout the visible spectrum, though it wouldbe obvious to those skilled in the art that a larger period beamsplitter would be useful in systems which are expected to display lessthan the full visible spectrum, such as just red, red and green, etc.

[0121] The performance of the wire grid beam splitter 14 is a functionof the element height or thickness t. The wire-grid height t must bebetween about 0.04 and 0.5 μm in order to provide the requiredperformance.

[0122] The performance of the wire grid beam splitter 14 is a functionof the width to period ratio (w/p) of the elements 30. The width w ofthe grid element 30 with respect to the period p must fall within theranges of approximately 0.3 to 0.76 in order to provide the requiredperformance.

[0123] The performance of the wire grid beam splitter 14 is a functionof the slopes of the sides of the elements 30. The slopes of the sidesof the grid elements 30 preferably are greater than 68 degrees fromhorizontal in order to provide the required performance.

[0124] As indicated above, other factors can effect the performanceand/or durability of the WG-PBS. For example, WG-PBS may be subjected tostrenuous optical environments, such as high flux illumination and otherphysically harsh conditions, for long periods of time, which can effectthe durability of the WG-PBS. Thus, it is desirable to protect theWG-PBS. As stated above, however, embedding the polarizer in a materialor medium with an index of refraction greater than one will alwayschange the performance of the polarizer over that available in air inthe same structure. Therefore, it is desirable to protect the polarizer,while optimizing its performance.

[0125] As illustrated in FIG. 9, an embedded wire grid polarizer of thepresent invention is shown, indicated generally at 200. The polarizer200 includes a first optical medium, material, layer or substrate 201; asecond optical medium, material or layer 203; and a plurality ofintervening elongated elements 205 sandwiched between the first andsecond layers 201 and 203. As indicated above, although certainadvantages are obtained by encasing or imbedding the elements, thepolarization or performance of the elements is detrimentally effected.Thus, the polarizer 10 of the present invention is designed to optimizethe performance when imbedded, as discussed below.

[0126] The first and second layers 201 and 203 have respective first andsecond surfaces 202 and 204 which face one another and the elements 205.The layers 201 and 203, or material of the layers, also have respectivefirst and second refractive indices. The first and second opticalmediums 201 and 203 each have a thickness t_(L1) and t_(L2), and areconsidered to be thick in an optical sense. They may be, for example,sheets of glass or polymer, an optical quality oil or other fluid, orother similar optical materials. The thickness t_(L1) or t_(L2) may beanywhere from a few microns to essentially infinite in extent.Preferably, the thickness t_(L1) and t_(L2) of the layers 201 and 203 isgreater than 1 micron. The optical media 201 and 203 may be the samematerials, such as two sheets of glass, or they may be chosen to bedifferent materials, such as an oil for material 203 and glass formaterial 201. The elements 205 may be supported by the first layer orthe substrate 201.

[0127] The intervening array of elements 205 includes a plurality ofparallel, elongated, spaced-apart, conductive elements 205. The elements205 have first and second opposite surfaces 205 a and 205 b, with thefirst surfaces 205 a facing towards the first surface 202 or first layer201, and second surfaces 205 b facing towards the second surface 204 orsecond layer 203. The first surfaces 205 a of the elements 205 maycontact and be coupled to the first surface 202 of the first layer 201,while the second surfaces 205 b may contact and be coupled to the secondsurface 204 of the second layer 203, as shown in FIG. 9. The array ofelements 205 is configured to interact with electromagnetic waves oflight in the visible spectrum to generally reflect most of the light ofa first polarization, and transmit most of the light of a secondpolarization.

[0128] The dimensions of the elements 205, and the dimensions of thearrangement of elements 205, are determined by the wavelength used, andare tailored for broad or full spectrum visible light. The elements 205are relatively long and thin. Preferably, each element 205 has a lengththat is generally larger than the wavelength of visible light. Thus, theelements 205 have a length of at least approximately 0.7 μm (micrometersor microns). The typical length, however, may be much larger. Inaddition, the elements 205 are located in generally parallel arrangementwith the spacing, pitch, or period P of the elements smaller than thewavelength of light. Thus, the pitch will be less than 0.4 μm(micrometers or microns).

[0129] The period of the elements 205, and the choices of the materialsfor the optical mediums 201 and 203 are all made to obtain and enhancethe desired interactions with the light rays 209, 211 and 213. The rayof light 209 is typically an unpolarized beam of light containingroughly equal amounts of the two polarizations known in the field as Spolarization and P polarization. However, the ray of light 209 may bealtered in specific applications to be partially or mostly of eitherpolarization as well. The period P of the elements 205 is chosen suchthat the wire grid will specularly reflect most of the light of the Spolarized light 211, and transmit most of the P polarized light 213.

[0130] The optical materials also are chosen to aid in this process. Forexample, it is possible to choose optical material 201 to be equallytransmissive to both S and P polarizations, while choosing an opticalmaterial 203 that would absorb S polarized light or otherwise aid in thetransmission of P polarized light and the reflection of S polarizedlight. In the preferred embodiment, the optical material composing thelayers 201 and 203 is glass. Other materials are also suitable dependingon the particular application. For example, the second layer 203 can bea sheet of glass or plastic. In addition the second layer can be a layerof vacuum deposited film, or optical thin film, such as silicon dioxide,silicon nitride, magnesium fluoride, titanium oxide, etc. The secondlayer 203 also can be formed by chemically treating the surface of theelements and first layer to leave a thin film, such as hexamethyldisilazane. Such a layer can be one or several atomic monolayers that isless sensitive to the environmental conditions. Alternatively, thischemical treatment may be chosen to actively impede the physicalmechanisms which cause the damage to the wire grid structure in theharsh environment. The second layer also can include a plurality offilms of material mentioned herein.

[0131] The intervening elongated elements 205 are not very large. Theywill typically be arranged in a regular, ordered array having a period Pon the order of 0.3 μm or less, with the width w_(R) of the ribs 205 andthe width w_(S) of the spaces or gaps 207 separating the elements on theorder of 0.15 μm or less. The width of the elements 205 and the spaces207 can be varied to achieve desired optical performance effects, whichwill be further described below. The height or thickness t_(R) of theseelements 205 will typically be between that required for the elements tobe optically opaque (approximately 40 nm in the case of aluminum) up toa height of perhaps 1 μm. The upper bound is fixed by considerations ofmanufacturing practicality as well as optical performance. In thepreferred embodiment, the elements 205 are typically composed ofmaterials such as aluminum or silver if the polarizer is to be usedacross the entire visible spectrum. However, if it is only required in aparticular case to provide a polarizer which performs in a portion ofthe spectrum, such as in red light, then other materials such as copperor gold could be used.

[0132] An important factor to obtaining the optimum performance of theimbedded wire grid polarizer 200 is the material disposed within thespaces or gaps 207. The gaps 207, formed between the elements 205,advantageously provide a refractive index less than the refractive indexof at least one of the layers 201 and 203, such as the first layer 201.Applicants have found that, when the gaps 207 provide a lower refractiveindex, the performance of the polarizer 200 is improved over wire gridstotally encapsulated in a material with a constant refractive index. Inthe preferred embodiment, this material will be air or vacuum, but forreasons of practicality or performance in certain applications, othermaterials may be used.

[0133] It is desirable that this material have the lowest refractiveindex n possible while meeting other necessary design constraints suchas manufacturability. These other constraints may require that thematerial filling the spaces 207 between the elongated elements 205 bethe same material as that composing either or both of the opticalmaterials 201 and 203. Or, the material filling the spaces 207 betweenthe elongated elements 205 may be chosen to be a material different fromthe optical materials 201 and 203.

[0134] As mentioned, in the preferred embodiment, the material in thespaces 207 will be air or vacuum. Other materials which may be usedinclude water (index 1.33), magnesium fluoride (index 1.38) or othercommon optical thin film materials which can be deposited usingevaporation, sputtering, or various chemical vapor deposition processes,optical oils, liquid hydrocarbons such as naptha, toluene, etc. or othermaterials with suitably low indices. The material in the gaps 207 alsocan include plastics, or fluorinated hydrocarbons (Teflon).

[0135] In addition, the substrate 201 (FIG. 9) or 40 (FIG. 7) of theWG-PBS can affect the performance of the WG-PBS. As stated above,orienting the substrate of the WG-PBS at an angle with respect to thelight can result in aberrations of astigmatism and coma whennon-collimated light passes therethrough. These aberrations occurregardless of the flatness of the substrate. Therefore, flat platetransmissive optics cannot be used in imaging applications unless theaberrations are corrected or rendered negligible.

[0136] This problem can be avoided in an imaging application if the beamof light containing the image is reflected from the front surface of theplate rather than passed through it, because it is the transit throughthe tilted substrate that causes the optical aberrations, such as alateral shift in position along the sloping direction of the substrate.Such a configuration requires a flat substrate in order to avoiddistortions in the beam that result in distortions in the final image.Depending on the application, flatness less than approximately 3standard wavelengths deviation per inch is preferable; less thanapproximately 1 standard wavelength deviation per inch is morepreferable; and less than {fraction (1/10)}^(th) standard wavelengthsdeviation per inch is most preferable.

[0137] For the transmitted beam, it is desirable to reduce astigmatismand chromatic aberration. Thus, the substrate preferably is very thin,or has a thickness less than approximately 5 millimeters.

[0138] Another important consideration for the image system of thepresent invention is to have a transmitted wave from the WG-PBS besufficiently free of geometric distortions. For the transmitted beam,the distortions are typically caused by deviations from parallel betweenthe two surfaces of the substrate. It is preferable that the transmittedbeam have a geometric distortion less than approximately 3 standardwavelengths deviation per inch; more preferably less than approximately½ standard wavelength deviation per inch; and most preferably less thanapproximately {fraction (1/10)}^(th) standard wavelength deviation perinch.

[0139] It is to be understood that the above-described arrangements areonly illustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the present invention and the appended claims are intendedto cover such modifications and arrangements. Thus, while the presentinvention has been shown in the drawings and fully described above withparticularity and detail in connection with what is presently deemed tobe the most practical and preferred embodiment(s) of the invention, itwill be apparent to those of ordinary skill in the art that numerousmodifications, including, but not limited to, variations in size,materials, shape, form, function and manner of operation, assembly anduse may be made, without departing from the principles and concepts ofthe invention as set forth in the claims.

What is claimed is:
 1. An image projection system, comprising: a) alight source capable of producing a visible light beam; b) a polarizingbeam splitter, located near the light source in the light beam andoriented at an angle with respect to the light beam, the beam splittercomprising: 1) a first transparent substrate having a first surfacelocated in the light beam with the light beam striking the first surfaceat an angle, and having a refractive index; 2) a second layer, separatefrom the first transparent substrate, having a refractive index; and 3)a generally parallel arrangement of thin, elongated, spaced-apartelements disposed between the first transparent substrate and the secondlayer, and forming a plurality of gaps between the elements, the gapsproviding a refractive index less than the refractive index of the firsttransparent substrate or the second layer, the arrangement beingconfigured and the elements being sized to interact with electromagneticwaves of the source light beam to generally (i) transmit light throughthe elements which has a polarization oriented perpendicular to a planethat includes at least one of the elements and the direction of theincident light beam, defining a transmitted beam, and (ii) reflect lightfrom the elements which has a polarization oriented parallel with theplane that includes at least one of the elements and the direction ofthe incident light beam, defining a reflected beam; c) a reflectivearray located near the polarizing beam splitter in either the reflectedor transmitted beam, the array modulating the polarization of the beamby selectively altering the polarization of the beam to encode imageinformation thereon and creating a modulated beam, the array beingoriented to direct the modulated beam back towards the polarizing beamsplitter; d) the beam splitter further being located in the modulatedbeam and oriented at an angle with respect to the modulated beam, andthe arrangement of elements of the beam splitter interacting withelectromagnetic waves of the modulated beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to the plane that includes at least one of the elementsand the direction of the incident light beam, defining a secondtransmitted beam, and (ii) reflect light from the elements which has apolarization parallel with the plane that includes at least one of theelements and the direction of the incident light beam, defining a secondreflected beam, to separate out the unaltered polarization from themodulated beam; e) a screen located in either the second reflected beamor the second transmitted beam for displaying the encoded imageinformation.
 2. A system in accordance with claim 1, wherein thetransparent substrate has a thickness less than approximately 5millimeters.
 3. A system in accordance with claim 1, wherein thetransparent substrate has a flatness less than approximately 3 standardwavelengths deviation per inch.
 4. A system in accordance with claim 1,wherein the first transmitted beam has a geometric distortion less thanapproximately 3 wavelengths deviation per inch.
 5. A system inaccordance with claim 1, wherein the gaps between the elements includeair.
 6. A system in accordance with claim 1, wherein the gaps betweenthe elements have a vacuum.
 7. A system in accordance with claim 1,wherein the gaps between the elements include a material different frommaterials of the first transparent substrate and the second layer.
 8. Asystem in accordance with claim 1, wherein the gaps include a materialthat is a same material as the second layer.
 9. A system in accordancewith claim 1, wherein the gaps include a material that is a samematerial as the first transparent substrate.
 10. A system in accordancewith claim 1, wherein the gaps between the elements include water.
 11. Asystem in accordance with claim 1, wherein the gaps between the elementsinclude magnesium fluoride.
 12. A system in accordance with claim 1,wherein the gaps between the elements include oil.
 13. A system inaccordance with claim 1, wherein the gaps between the elements includehydrocarbon compounds.
 14. A system in accordance with claim 1, whereinthe gaps between the elements include plastic.
 15. A system inaccordance with claim 1, wherein the gaps between the elements includefluorinated hydrocarbon.
 16. A system in accordance with claim 1,wherein the arrangement has a configuration and the elements have a sizewhich would normally create a resonance effect in combination with oneof the layer or the substrate within the visible spectrum; and whereinthe gaps with a lower refractive index than the refractive index of oneof the layer or the substrate causes a shift of the normally occurringresonance effect to a lower wavelength, thereby broadening a band ofvisible wavelengths in which no resonance effect occurs.
 17. A system inaccordance with claim 1, wherein the second layer includes a film.
 18. Asystem in accordance with claim 1, wherein the second layer includes aplurality of films.
 19. A system in accordance with claim 1, wherein thesecond layer includes a vacuum deposited film selected from the groupconsisting of: silicon dioxide, silicon nitride, magnesium fluoride, andtitanium oxide.
 20. A system in accordance with claim 1, wherein thesecond layer includes a sheet of glass.
 21. A system in accordance withclaim 1, wherein the second layer includes a sheet of plastic.
 22. Asystem in accordance with claim 1, wherein the second layer includes afilm of hexamethal disilazane.
 23. A system in accordance with claim 1,wherein the beam splitter is a generally planar sheet.
 24. A system inaccordance with claim 1, wherein the beam splitter is oriented withrespect to the light beam or the modulated beam at an incident anglebetween approximately 0 to 80 degrees.
 25. A system in accordance withclaim 1, wherein the beam splitter is oriented with respect to the lightbeam or the modulated beam at incidence angles greater than 47 degreesor less than 43 degrees.
 26. A system in accordance with claim 1,wherein the light beam has a useful divergent cone with a half anglebetween approximately 10 and 25°.
 27. A system in accordance with claim1, wherein the beam splitter is used at an F-number less thanapproximately f/2.5.
 28. A system in accordance with claim 1, whereinthe beam splitter has a throughput of at least 50% defined by theproduct of the fractional amount of p-polarization transmitted light andthe fractional amount of s-polarization reflected light; and wherein thes-polarization transmitted light and p-polarization reflected light areboth less than 5%.
 29. A system in accordance with claim 1, wherein thebeam splitter has a throughput of at least 50% defined by the product ofthe fractional amount of s-polarization transmitted light and thefractional amount of p-polarization reflected light; and wherein thep-polarization transmitted light and s-polarization reflected light areboth less than 5%.
 30. A system in accordance with claim 1, wherein thebeam splitter has a throughput for the light beam of at least 65%,defined by the product of the fractional amount of reflected light andthe fractional amount of transmitted light; and wherein the percent ofreflected light or the percent of transmitted light is greater thanapproximately 67%.
 31. A system in accordance with claim 1, furthercomprising a pre-polarizer disposed between the light source and thebeam splitter.
 32. A system in accordance with claim 1, furthercomprising a post-polarizer disposed between the beam splitter and thescreen.
 33. A system in accordance with claim 1, wherein the array isdisposed in the reflected beam; and wherein the screen is disposed inthe second transmitted beam.
 34. A system in accordance with claim 1,wherein the array is disposed in the transmitted beam; and wherein thescreen is disposed in the second reflected beam.
 35. A system inaccordance with claim 1, wherein a) the arrangement of elements has aperiod less than approximately 0.21 microns, b) the elements have athickness between approximately 0.04 to 0.5 microns, and c) the elementshave a width of between approximately 30 to 76% of the period.
 36. Asystem in accordance of claim 1, wherein the elements each have a crosssection with a base, a top opposite the base, and opposite left andright sides; and wherein the sides form an angle with respect to thebase greater than approximately 68 degrees.
 37. A method for projectingan image, the method comprising: a) producing a source light beam havinga wavelength in a range between approximately 0.4 to 0.7 microns using alight source; b) substantially separating polarizations of the sourcelight beam using a polarizing beam splitter disposed in the source lightbeam, the polarizing beam splitter including: 1) a first layer having arefractive index; 2) a second layer, separate from the first layer,having a refractive index; 3) a generally parallel arrangement of thin,elongated, spaced-apart elements, disposed between the first and secondlayers, configured and sized to interact with electromagnetic waves ofthe source light beam to generally (i) transmit light through theelements which has a polarization oriented perpendicular to a plane thatincludes at least one of the elements and the direction of the incidentlight beam, defining a transmitted beam, and (ii) reflect light from theelements which has a polarization orientation that lies in the planethat includes at least one of the elements and the direction of theincident light beam, defining a reflected beam; 4) a plurality of gaps,formed between the elements and the first and second layers, configuredto provide a refractive index less than the refractive index of thefirst or second layers; c) modulating either the transmitted orreflected beam and creating a modulated beam by selectively altering thepolarization of the beam using an array disposed in either thetransmitted or reflected beam; d) substantially separating thepolarizations of the modulated beam using the polarizing beam splitterdisposed in the modulated beam, the elements interacting withelectromagnetic waves of the modulated beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to plane that includes at least one of the elements andthe direction of the incident light beam, defining a second transmittedbeam, and (ii) reflect light from the elements which has a polarizationorientation that lies in the plane that includes at least one of theelements and the direction of the incident light beam, defining a secondreflected beam; and e) displaying either the second transmitted beam orthe second reflected beam on a screen.
 38. An image display system forproducing a visible image, the system comprising: a) a light sourceconfigured to emit a source light beam having a wavelength in a rangebetween approximately 0.4 to 0.7 microns; b) a liquid crystal arraypositioned and oriented to receive and modulate at least a portion ofthe source light beam to create a modulated beam containing imageinformation; c) a screen positioned and oriented to receive and displayat least a portion of the modulated beam; and d) a polarizing beamsplitter positioned and oriented to receive both the source light beamand the modulated beam, the polarizing beam splitter including: 1) afirst layer having a refractive index; 2) a second layer, separate fromthe first layer, having a refractive index; 3) a generally parallelarrangement of thin, elongated, spaced-apart elements, disposed betweenthe first and second layers, configured and sized to interact withelectromagnetic waves of the source light beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to a plane that includes at least one of the elements andthe direction of the incident light beam, defining a transmitted beam,and (ii) reflect light from the elements which has a polarizationorientation that lies in the plane that includes at least one of theelements and the direction of the incident light beam, defining areflected beam, and interacts with the electromagnetic waves of themodulated beam to generally (i) transmit light through the elementswhich has a polarization oriented perpendicular to a plane that includesat least one of the elements and the direction of the modulated lightbeam, defining a second transmitted beam, and (ii) reflect light fromthe elements which has a polarization orientation that lies in the planethat includes at least one of the elements and the direction of themodulated light beam, defining a second reflected beam; and 4) aplurality of gaps, formed between the elements and the first and secondlayers, configured to provide a refractive index less than therefractive index of the first or second layers.
 39. A system inaccordance with claim 38, wherein the first layer is a substrate havinga thickness less than approximately 5 millimeters.
 40. A system inaccordance with claim 38, wherein the first layer is a substrate havinga flatness less than approximately 3 standard wavelengths deviation perinch.
 41. A system in accordance with claim 38, wherein the either ofthe first or second transmitted beams has a geometric distortion lessthan approximately 3 wavelengths deviation per inch.
 42. A system inaccordance with claim 38, wherein the gaps between the elements includeair.
 43. A system in accordance with claim 38, wherein the gaps betweenthe elements have a vacuum.
 44. A system in accordance with claim 38,wherein the gaps between the elements include a material different frommaterials of the first transparent substrate and the second layer.
 45. Asystem in accordance with claim 38, wherein the gaps include a materialthat is a same material as the second layer.
 46. A system in accordancewith claim 38, wherein the gaps include a material that is a samematerial as the first transparent substrate.
 47. A system in accordancewith claim 38, wherein the gaps between the elements include water. 48.A system in accordance with claim 38, wherein the gaps between theelements include magnesium fluoride.
 49. A system in accordance withclaim 38, wherein the gaps between the elements include oil.
 50. Asystem in accordance with claim 38, wherein the gaps between theelements include hydrocarbon compounds.
 51. A system in accordance withclaim 38, wherein the gaps between the elements include plastic.
 52. Asystem in accordance with claim 38, wherein the gaps between theelements include fluorinated hydrocarbon.
 53. A system in accordancewith claim 38, wherein the arrangement has a configuration and theelements have a size which would normally create a resonance effect incombination with one of the layers within the visible spectrum; andwherein the gaps with a lower refractive index than the refractive indexof one of the layers causes a shift of the normally occurring resonanceeffect to a lower wavelength, thereby broadening a band of visiblewavelengths in which no resonance effect occurs.
 54. A system inaccordance with claim 38, wherein the second layer includes a film. 55.A system in accordance with claim 38, wherein the second layer includesa plurality of films.
 56. A system in accordance with claim 38, whereinthe second layer includes a vacuum deposited film selected from thegroup consisting of: silicon dioxide, silicon nitride, magnesiumfluoride, and titanium oxide.
 57. A system in accordance with claim 38,wherein the second layer includes a sheet of glass.
 58. A system inaccordance with claim 38, wherein the second layer includes a sheet ofplastic.
 59. A system in accordance with claim 38, wherein the secondlayer includes a film of hexamethal disilazane.
 60. A system inaccordance with claim 38, wherein a) the arrangement of elements has aperiod less than approximately 0.21 microns, b) the elements have athickness between approximately 0.04 to 0.5 microns, and c) the elementshave a width of between approximately 30 to 76% of the period.
 61. Asystem in accordance with claim 38, wherein the array is disposed in thereflected beam, and wherein the screen is disposed in the secondtransmitted beam.
 62. A system in accordance with claim 38, wherein thearray is disposed in the transmitted beam, and wherein the screen isdisposed in the second reflected beam.
 63. An image projection system,comprising: a) a light source producing a visible light beam; b) apolarizing beam splitter located near the light source in the light beamand oriented at an angle with respect to the light beam, the beamsplitter comprising: 1) a first transparent substrate having a firstsurface located in the light beam with the light beam striking the firstsurface at an angle; 2) the first transparent substrate having athickness less than approximately 5 millimeters; and 3) a generallyparallel arrangement of thin, elongated, spaced-apart elements, disposedon the first transparent substrate, the arrangement being configured andthe elements being sized to interact with electromagnetic waves of thesource light beam to generally (i) transmit light through the elementswhich has a polarization oriented perpendicular to a plane that includesat least one of the elements and the direction of the incident lightbeam, defining a transmitted beam, and (ii) reflect light from theelements which has a polarization oriented parallel with the plane thatincludes at least one of the elements and the direction of the incidentlight beam, defining a reflected beam; c) a reflective array locatednear the polarizing beam splitter in either the reflected or transmittedbeam, the array modulating the polarization of the beam by selectivelyaltering the polarization of the beam to encode image informationthereon and creating a modulated beam, the array being oriented todirect the modulated beam back towards the polarizing beam splitter; d)the beam splitter further being located in the modulated beam andoriented at an angle with respect to the modulated beam, and thearrangement of elements of the beam splitter interacting withelectromagnetic waves of the modulated beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to the plane that includes at least one of the elementsand the direction of the incident light beam, defining a secondtransmitted beam, and (ii) reflect light from the elements which has apolarization parallel with the plane that includes at least one of theelements and the direction of the incident light beam, defining a secondreflected beam, to separate out the unaltered polarization from themodulated beam; and e) a screen located in either the second reflectedbeam or the second transmitted beam for displaying the encoded imageinformation.
 64. A system in accordance with claim 63, furthercomprising: a) second layer, separate from the first transparentsubstrate; and b) the arrangement of elements being disposed between thefirst transparent substrate and the second layer; and c) a plurality ofgaps formed between the elements; and d) the gaps providing a refractiveindex less than a refractive index of the first transparent substrate orthe second layer.
 65. A system in accordance with claim 63, wherein thefirst transparent substrate having a flatness less than approximately 3standard wavelengths deviation per inch.
 66. A system in accordance withclaim 63, wherein either the first or second transmitted beams have ageometric distortion less than approximately 3 wavelengths deviation perinch.
 67. An image projection system, comprising: a) a light sourceproducing a visible light beam; b) a polarizing beam splitter locatednear the light source in the light beam and oriented at an angle withrespect to the light beam, the beam splitter comprising: 1) a firsttransparent substrate having a first surface located in the light beamwith the light beam striking the first surface at an angle; 2) the firsttransparent substrate having a flatness less than approximately 3standard wavelengths deviation per inch; and 3) a generally parallelarrangement of thin, elongated, spaced-apart elements, disposed on thefirst transparent substrate, the arrangement being configured and theelements being sized to interact with electromagnetic waves of thesource light beam to generally (i) transmit light through the elementswhich has a polarization oriented perpendicular to a plane that includesat least one of the elements and the direction of the incident lightbeam, defining a transmitted beam, and (ii) reflect light from theelements which has a polarization oriented parallel with the plane thatincludes at least one of the elements and the direction of the incidentlight beam, defining a reflected beam; c) a reflective array locatednear the polarizing beam splitter in either the reflected or transmittedbeam, the array modulating the polarization of the beam by selectivelyaltering the polarization of the beam to encode image informationthereon and creating a modulated beam, the array being oriented todirect the modulated beam back towards the polarizing beam splitter; d)the beam splitter further being located in the modulated beam andoriented at an angle with respect to the modulated beam, and thearrangement of elements of the beam splitter interacting withelectromagnetic waves of the modulated beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to the plane that includes at least one of the elementsand the direction of the incident light beam, defining a secondtransmitted beam, and (ii) reflect light from the elements which has apolarization parallel with the plane that includes at least one of theelements and the direction of the incident light beam, defining a secondreflected beam, to separate out the unaltered polarization from themodulated beam; and e) a screen located in either the second reflectedbeam or the second transmitted beam for displaying the encoded imageinformation.
 68. A system in accordance with claim 67, furthercomprising: a) a second layer, separate from the first transparentsubstrate; and b) the arrangement of elements being disposed between thefirst transparent substrate and the second layer; and c) a plurality ofgaps formed between the elements; and d) the gaps providing a refractiveindex less than a refractive index of the first transparent substrate orthe second layer.
 69. A system in accordance with claim 67, wherein thefirst transparent substrate has a thickness less than approximately 5millimeters.
 70. A system in accordance with claim 67, wherein eitherthe first or second transmitted beams have a geometric distortion lessthan approximately 3 wavelengths deviation per inch.
 71. An imageprojection system, comprising: a) a light source producing a visiblelight beam; b) a polarizing beam splitter located near the light sourcein the light beam and oriented at an angle with respect to the lightbeam, the beam splitter comprising: 1) a first transparent substratehaving a first surface located in the light beam with the light beamstriking the first surface at an angle; and 2) a generally parallelarrangement of thin, elongated, spaced-apart elements, disposed on thefirst transparent substrate, the arrangement being configured and theelements being sized to interact with electromagnetic waves of thesource light beam to generally (i) transmit light through the elementswhich has a polarization oriented perpendicular to a plane that includesat least one of the elements and the direction of the incident lightbeam, defining a transmitted beam, and (ii) reflect light from theelements which has a polarization oriented parallel with the plane thatincludes at least one of the elements and the direction of the incidentlight beam, defining a reflected beam; c) a reflective array locatednear the polarizing beam splitter in either the reflected or transmittedbeam, the array modulating the polarization of the beam by selectivelyaltering the polarization of the beam to encode image informationthereon and creating a modulated beam, the array being oriented todirect the modulated beam back towards the polarizing beam splitter; d)the beam splitter further being located in the modulated beam andoriented at an angle with respect to the modulated beam, and thearrangement of elements of the beam splitter interacting withelectromagnetic waves of the modulated beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to the plane that includes at least one of the elementsand the direction of the incident light beam, defining a secondtransmitted beam, and (ii) reflect light from the elements which has apolarization parallel with the plane that includes at least one of theelements and the direction of the incident light beam, defining a secondreflected beam, to separate out the unaltered polarization from themodulated beam; and e) a screen located in either the second reflectedbeam or the second transmitted beam for displaying the encoded imageinformation; and f) the first and second transmitted beams having ageometric distortion less than approximately 3 wavelengths deviation perinch.
 72. A system in accordance with claim 71, further comprising: a) asecond layer, separate from the first transparent substrate; and b) thearrangement of elements being disposed between the first transparentsubstrate and the second layer; and c) a plurality of gaps formedbetween the elements; and d) the gaps providing a refractive index lessthan a refractive index of the first transparent substrate or the secondlayer.
 73. A system in accordance with claim 71, wherein the firsttransparent substrate has a thickness less than approximately 5millimeters.
 74. A system in accordance with claim 71, wherein the firsttransparent substrate having a flatness less than approximately 3standard wavelengths deviation per inch.